Audio service: only duck started players + refactor

Refactor management of list of ducked players:
  DuckingManager has a list of DuckedApps, which reference
  the ducked players per uid.
Only consider ducking a player when it is in STARTED state.
When a player is released, remove it from the list of ducked players.

Test: play audio in GPM while having driving directions, music ducks
Bug: 37433811
Change-Id: I038a963432c0df6c9470a3a4fb80049d55e8719c